Multi-effect distillation (MED) process has been used in industry for juice evaporation, to concentrate a substance, for production of salts and for salty and marine water distillation for fresh water production. Different processes have been used worldwide for desalination, for fresh water production. Major processes commercially available are membranes (reverse osmosis and electrodialysis) and thermal. Distillation is a thermal process that can be divided in three different methods: multi-stage flash distillation (MFD); multi-effect distillation (MED) and vapor compression. These processes can be used also to concentrate a substance as the object purpose.
In the MED process, only a portion of the concentrate submitted to the heat transfer surfaces is evaporated. Each effect works in a specific equilibrium vapor pressure state. The remaining liquid of each effect, normally called brine, is the entrance feed to the next stage, where part of it flashes into vapor.
Produced vapor in one effect will give up heat to boil the liquid transferred to the next effect due to the temperature difference between them, and several constructive models have been based on the type of evaporators used and on the creative design and arrangement distinctly disclosed in many patents worldwide.
Sometimes the effects or stages have evaporators located in separate vessels, having the disadvantages of requiring a pipeline for conducting vapor from one stage to the next, and the necessity for more room, as shown in the U.S. Pat. Nos. 3,884,767, 3,261,766 and 3,021,265. When these stages are assembled in only one vessel, the construction can have the stages arranged in multi stack vertical falling film evaporators as disclosed in the U.S. Pat. Nos. 4,334,954, 6,089,312, 6,309,513, 3,487,873, and 6,089,312, all involving falling film type evaporators. Comment must be made to the Sephton (6,309,513) and Biar et al. (6,089,312) patents that are not multi-effect apparatus but a parallel stack of evaporators.
Vertical Tube Evaporators (VTE) have basically two different evaporation systems: falling film and rising film evaporation. As widely described in technical literature, falling film evaporators have high heat transfer coefficients, but the proper design of the liquid distribution system is critical to achieve full and even product wetting of the tubes, with higher risk of having so-called dry spots or film breakdown or vapor blanket, that causes a lowering of heat transfer and is the cause of plugging by scale.
Vapor compression process has the great advantage of a low energy consumption and a high energy efficiency, but has the disadvantage of higher maintenance costs associated with down-time operations per the rotary equipment involved, as the compressor and respective driver, and sometimes the whole evaporator as disclosed in the single stage apparatus of U.S. Pat. No. 6,695,951.
An exemplary two-stage embodiment of a multi-stage distillation apparatus includes a plurality of evaporators connected in series for staged operation in a rising film evaporation process wherein the evaporators are disposed in a compact concentric arrangement. The apparatus includes a first stage evaporator of ring shell and tube construction including a first annular vertical tube bundle, having tubes supported and sealed by a first upper tubesheet and a first bottom tubesheet. An external wall and an internal wall enclose the first tube bundle. The internal wall has an upward extension over the upper tubesheet and the external wall has a downward extension adjacent the bottom tubesheet and fastened to a base. The base includes a feed chamber communicating through the bottom tubesheet with the tubes of the first tube bundle for supplying the tubes with a flow of undistilled water for partial vaporization. The apparatus also includes means for conducting a heated liquid against the tubes of the first tube bundle and partially vaporizing the undistilled water therein. The first stage evaporator has a first stage vapor chamber above the upper tubesheet and in open communication with the tubes of the first tube bundle for receiving therefrom heated water vapor and residual undistilled water. The upward extension of the internal wall directs the heated vapor to a subsequent stage evaporator tube bundle. A final stage evaporator, which is also a pre-final stage condenser, includes a final cylindrical vertical tube bundle, having tubes supported and sealed by a final upper tubesheet and a final bottom tubesheet. The final bottom tubesheet carries a final stage floating head connected to receive, by gravity feed means from a final stage vapor chamber, residual undistilled water vacuumed from a prior stage evaporator to the final stage vapor chamber. The final floating head communicates with the tubes of the final stage evaporator cylindrical tube bundle for delivering the residual undistilled water there into. The final upper tubesheet has a diameter at least 30% larger than the final bottom tubesheet. The final upper tubesheet defines a lower wall of the final stage vapor chamber, in open communication with the final tube bundle for receiving therefrom additional water vapor for condensation to condensate and residual undistilled water for discharge from the distillation apparatus. A final external armor shell surrounds the final cylindrical tube bundle and engages the internal wall of an adjacent evaporator to direct heated vapor from the adjacent evaporator to pass through the final cylindrical tube bundle for heating the tubes and causing partial condensation of the heated vapor on the tubes of the final stage evaporator with the partial vaporization of the residual undistilled water in the tubes of the final tube bundle. Means for drawing off condensate from the partially condensed vapor from the final stage evaporator is also provided.
An exemplary three stage embodiment includes the first and final stage features of the two-stage version described above and adds a second stage evaporator and first stage condenser for heating the tubes and causing partial condensation of the heated vapor on the tubes of the final stage evaporator with the partial vaporization of the residual undistilled water in the tubes of the final tube bundle. The three stage embodiment further includes means for drawing off condensate from the partially condensed vapor from the final stage evaporator including a second intermediate vertical ring tube bundle having tubes supported and sealed by a second upper tubesheet and a second bottom tubesheet. The second bottom tubesheet carries a second stage floating head connected to receive, by gravity feed from a second stage vapor chamber above the second upper tubesheet, residual undistilled water vacuumed from the first stage evaporator to the second stage vapor chamber. The second stage floating head communicates with tubes of the second stage evaporator ring tube bundle for delivering the residual undistilled water thereinto. A second stage internal wall is welded to the second upper tubesheet. The second stage internal wall has an upward extension over the second upper tubesheet, and partially defining the second stage vapor chamber, which is open to communication with the second tube bundle for receiving therefrom additional water vapor for condensation to condensate and additional residual undistilled water. The second intermediate vertical ring tube bundle is disposed concentrically between the first annular vertical tube bundle of the shell and tube first stage evaporator and a subsequent vertical tube bundle of a subsequent evaporator. A second stage external armor shell surrounds the second ring tube bundle and engages the internal wall of the first stage evaporator to direct vapor from the first stage evaporator to pass through the second ring tube bundle for partial condensation of the heated vapor from the first stage tube bundle on the tubes of the second stage evaporator and the partial vaporization of the residual undistilled water in the tubes of the second ring tube bundle. Means for drawing off condensate from the partially condensed vapor from the second stage evaporator is also provided.
An exemplary four stage embodiment includes the first, second and final stage features of the three-stage version described above and adds a third stage evaporator and second stage condenser including a third intermediate vertical ring tube bundle, having tubes supported and sealed by a third upper tubesheet and a third bottom tubesheet. The third bottom tubesheet carries a third stage floating head connected to receive, by gravity feed means from a third stage vapor chamber above the third upper tubesheet, residual undistilled water vacuumed from the second stage evaporator to the third stage vapor chamber. The third stage floating head communicates with the tubes of the third stage evaporator ring tube bundle for delivering the residual undistilled water thereto. A third stage internal wall is welded to the third upper tubesheet. The third stage internal wall has an upward extension over the upper tubesheet, and partially defines the third stage vapor chamber. The third intermediate vertical ring tube bundle is disposed concentrically between the second intermediate vertical ring tube bundle and the final cylindrical vertical tube bundle of the final evaporator. A third stage external armor shell surrounds the third ring tube bundle and engages the second stage internal wall of the second stage evaporator to direct heated vapor from the second stage evaporator to pass through the third ring tube bundle for partial condensation of the heated vapor from the second stage evaporator on the tubes of the third ring tube bundle and the partial vaporization of the residual undistilled water in the tubes of the second ring tube bundle. Means for drawing off condensate from the partially condensed vapor from the third stage evaporator is also provided.
Intended to improve the performance and reduce the height dimensions of such distillers, the present invention was developed using rising film evaporators, in a multi-effect apparatus. The several evaporators are assembled in a concentric disposition, using a shell and tube exchanger for the first stage and a bundle of tubes for the succeeding stages, which are inserted one inside each other and connected in series on a horizontal base. If not developed on this disposition, this apparatus will need a pump to push sea water to the higher stages, and necessarily will have its dimensions increased upwardly and no reduction in height would be accomplished.
Through this constructive arrangement, the following advantages are achieved:                material reduction due to the absence of vapor pipelines;        vapor friction losses reduced to a minimum;        smaller size due to the compactness of the concentric disposition of evaporators;        no heat loss to exterior in the inner stages; and        cost effectiveness.        
This unit can also be used to concentrate a mixture, using low temperature evaporative process.
The figures attached, are representative of four different models, showing their respective stages, all using the same constructive arrangement, here named concentric evaporators. The higher the number of stages the lower the energy consumption per volume produced. The choice for the number of stages, depend on the available heat, the fresh water rate desired and of course the involved costs.
The unit can be designed to produce any desired flow rate, meanwhile it is usual for this kind of equipment to have a production flow rate ranging from 5 until 120 m3/d.
The dimensions of a two stage 60 m3/d distiller have approximately 2.2 m height and 1.2 m in diameter.